This invention relates to a predistortion linearizer circuit suitable for applying a signal to a power amplifier for linearizing a signal outputted by the amplifier and, more particularly, to a linearizing circuit having a reduced number of components and reduced physical size advantageous for satellite communication systems.
High power amplifiers are employed in numerous situations including satellite communication systems wherein a high power amplifier, such as a solid state power amplifier or a traveling wave tube amplifier, is employed to amplify a signal to a sufficient power level for transmission from the satellite to a ground station. A disadvantageous characteristic of the high power amplifiers is a changing gain as a function of input signal level and a changing phase shift as a function of input signal level. As a result, the requisite high power is obtained at the price of introduction of amplitude and phase nonlinearities to the signals transmitted via a satellite communication link. Thus, there is distortion in the information carrying RF (radio frequency) signal.
To counteract the distortion introduced by the power amplifier, it has been the practice to employ a predistortion linearizer circuit in the signal path before the amplifier to introduce a distortion to the signal, in the form of both an amplitude and a phase nonlinearity, which is the inverse of the amplifier distortion, thereby to cancel the amplifier distortion and provide a linear output signal. Most predistortion linearizer circuits described in the microwave circuit literature make use of power combing hybrids in conjunction with nonlinear circuits to realize either a reflective or transmission predistortion linearizer. These circuits are typically rather large due to their complexity. Some circuits are more compact and employ an .active elements such as a gallium-arsenide field-effect transistor (GaAs FET) in a common-gate or common-source configuration. Examples of such a circuit are provided in U.S. Pat. Nos. 5,038,113 (General Electric) and 5,138,275 (Siemens).
A problem arises in that the foregoing predistortion linearizer circuits are more complex and costly than is desired, and may not allow for independent control of amplitude and phase nonlinearities. The complexity is due in part to the need for a bias circuit to supply the requisite operating bias current and voltage to the transistor, and to the use of relatively complex additional circuitry to develop the desired nonlinearities.